As a measuring method for an amount of charge of toner, a blow-off method is known.
In the blow-off method, a binary developer composed of toner particles and carrier particles is placed on a retaining member having meshes which are bigger than a diameter of a toner particle and are smaller than a diameter of a carrier particle, and only the toner particles are blown off by dry air blow (for example, nitrogen). Then, an amount of charge of carrier remained on the retaining member is measured so that an amount of charge of toner is obtained by the weight.
However, with the blow-off method, since a measured value changes with flow of dry air blow and a time required for measurement, it is difficult to measure an accurate amount of charge of toner. Moreover, distribution of the amount of charge cannot be obtained. Further, since an unary developer does not include carriers, an amount of charge of toner cannot be theoretically measured by using the unary developer.
The Japanese Unexamined Patent Publication No. 79958/1982 (Tokukaisho 57-79958) discloses the following method. Toner particles are introduced into a chamber where air blow has been sent at an uniform speed and an uniform electric field has been formed, the toner particles which have passed through the electric field are accumulated on a sheet and distribution of the toner particle on the sheet is measured so that distribution of a relative amount of charge of toner is obtained.
However, in the above method, since it is necessary to take out sheets where the toner particles are accumulated for every measurement, there exists a problem that successive measurement cannot be made. Moreover, there exists a problem that an amount of charge of each toner particle cannot be measured.
The Japanese Unexamined Patent Publication No. 277071/1986 (Tokukaisho 61-277071) discloses the following method. Toner particles are allowed to freely fall to a vertical passage where an electric field has been formed, a laser beam is irradiated to toner which passes a measurement position and a speed of the toner particles is measured from Doppler frequency of a scattered light so that an amount of charge and distribution of the amount of charge are obtained.
However, in this method, since momentum of the toner particles is small, the toner particles adhere to a wall surface of the passage. Therefore, there exists a problem that the toner particles is difficult to pass the measurement position. Furthermore, since toner is usually black, if a strong laser beam is not irradiated, a scattered light having strength necessary for measurement cannot be obtained. For this reason, since a large-size laser such as a He--Ne laser, or an Ar laser is required, there exists a problem that a measuring device becomes larger.
In addition, Japanese Unexamined Patent Publication No. 25772/1992 (Tokukaihei 4-25772) discloses the following method. Toner particles which fly at uniform flow velocity in an electric field are observed by using a magnifying optical system, and a diameter and displacement of the toner particles are obtained by image-processing observed data so that an amount of charge of toner is obtained.
However, in this method, since an observation range is narrow, it is necessary to scan an observation region in order to observe all the toner particles. For this reason, there exists the same problem as in the above-mentioned methods that a measuring device becomes larger.
As mentioned above, since a measuring device is large even if any of the above methods is used, there exists a problem that the measuring device is difficult to be installed in an image forming apparatus, such as a copying machine. Moreover, since toner particles are extracted as sample so that an amount of charge of the sample is measured, there exists a problem that an amount of charge of toner at the time of image formation cannot be obtained.